1. Field of the Invention
This invention relates to a method of producing carbon materials, and more particularly to a method of producing a high density and high strength carbon material having a bulk density of 1.45-2.05 by starting from a calcined product having a benzene insoluble matter (BI) of more than 95% by weight, a quinoline insoluble matter (QI) of more than 80% by weight and a volatile matter (VM) of 4-15% by weight, which is prepared by subjecting coal tar pitch to heat treatment, solvent extraction, filtration and calcination in this order, or a classified product thereof and then firing and graphitizing it without using a binder.
The carbon materials produced by the invention are utilized for wide applications such as electrodes for steel-making, electric brushes, materials for nuclear reactor, materials for machine and the like because they have various advantages and merits, namely, that they are a good conductor for electricity and heat, are stable at high temperature in a non-oxidizing atmosphere, large in the hot strength, difficult to be attacked by acid, alkali or other chemicals, easy in the machining, excellent in the self-lubrication, and small in the absorption area against thermal neutron and excellent in the slowing-down power.
2. Description of the Prior Art
In general, the carbon materials of this type are produced by mixing filler coke with a binder, molding the mixture, and firing or graphitizing the molded body. However, they are required to have many properties in accordance with the intended use involved and extending over a wide range, (for example, from materials for machine and special electrode materials . . . requiring high density and strength as far as possible . . . to electric brushes requiring relatively low density and strength and a high electrical resistivity).
As a method of producing such carbon materials in accordance with the intended use, it has hitherto been required to take very complicated production steps by changing the kind of the filler coke or the binder (or the mixing ratio of filler coke to binder) or by adding other additives such as natural graphite and the like to the mixture of filler coke and binder. That is, the aforementioned mixing of filler coke and binder complicates the working steps and degrades the working environment. Further, the filler coke itself is porous, while the coal tar pitch or synthetic resin to be used as the binder produces a large amount of pores in the firing. As a result, the above method merely produces carbon materials having a density of about 1.7 g/cm.sup.3 and a bending strength of 500-600 kg/cm.sup.2 at most, and it is difficult to produce carbon materials having higher density and strength. In addition, since the filler coke usually has an anisotropy, the resulting carbon material has also an anisotropy, so that it is difficult to produce carbon materials having isotropic properties.
Lately with the industrial growth, the demands for higher quality of the carbon material have become severe. As a result, various needs around carbon materials (having an isotropy and high density and strength) are rapidly increasing. In response to such needs, the development of carbon materials having the isotropy and high density and strength is taking place extensively in various fields. In Japanese Patent laid open No. 49-23,791 or Japanese Patent Application Publication No. 51-29,523, for instance, there is proposed a method of producing isotropic high density carbon materials, wherein optically anisotropic mesophase spheres (obtained by heating pitches as a raw material at a temperature of 350.degree.-500.degree. C.) are separated from pitch matrix with a solvent, molded under pressure without a binder, fired and graphitized in the usual manner.
In this conventional method, however, quinoline having a strong extractability is used for the solvent separation of optically anisotropic mesophase spheres, so that pitch fractions are removed from the optically anisotropic mesophase sphere and consequently the binding force of the optically anisotropic mesophase sphere itself is weakened and the high densification thereof is difficult. Furthermore, the fractionation in the solvent separation must be performed by repeatedly using various solvents in addition to quinoline, so that some of low-boiling pitch fraction and solvents are liable to still remain in the resulting optically anisotropic mesophase spheres, and the molded body produced from such mesophase spheres is apt to cause cracking and blistering phenomena resulted from the remaining low-boiling pitch fraction and solvent at the firing step. Therefore, the above prior art has many problems as the industrial production method.